1. Field of the Invention
The present invention relates to an optical device molding method whereby an optical device made of a glass such as lens, prism, or the like is manufactured by press-molding a glass preform.
2. Related Background Art
A technique for manufacturing a lens by press-molding a glass preform without needing the grinding process eliminates complicated processes which are needed in the conventional manufacturing method of a lens and can easily cheaply manufacture the lens. In recent years, such a technique has been used to manufacture not only a lens but also a prism and other optical devices made of glass.
(1) Hardness can be mentioned as a property which is required of a die material which is used to press-mold such optical devices made of glass or the like. Hitherto, as such kinds of die materials, many materials such as metal, ceramics, material on which metal or ceramics are coated, and the like have been proposed. For instance, a 13 Cr martensite steel has been proposed in JP-A-49-51112, and SiC and Si.sub.3 N.sub.4 have been proposed in JP-A-52-45613.
(2) In an Official Gazette of an apparatus for molding glass in JP-U-42-8123 (based on the priority on Dec. 28, 1961; U.S. patent Ser. No. 162743, Applicant is General Precision Inc.), the following apparatuses have been disclosed.
A glass substance molding apparatus comprising a combination of: a sealed chamber having an inert atmosphere; a metal die which is disposed in the sealed chamber and has a cavity in the die; a glass substance put in the cavity; and an apparatus for heating the glass substance to flow the glass and to fill in the cavity.
An apparatus according to the above, wherein the metal of the die has a expansion coefficient lower than an expansion coefficient of glass.
An apparatus for molding a glass substance comprising a combination of: a sealed chamber having an inert atmosphere; a die of a metal portion which is supported by this chamber and has a cavity in the die; a plunger for applying a continuous pressure when one end is put into the cavity which is slidably attached onto the die connected to the cavity; and a heating apparatus for heating glass in the cavity.
An apparatus according to the former apparatus, wherein the heating apparatus includes a heat sink apparatus which is arranged on the outside of the sealed chamber and heats the glass by the induction and surrounds the metal die.
(3) In the specifications of U.S. Pat. Nos. 833347 and 3900328 (JP-B-54-38126), there is disclosed a manufacturing method in which a glass-like carbon die is disposed in a proper chamber and which comprises the steps of: (a) inserting a glass block into a cavity of the die; (b) evacuating the chamber; (c) degassing the die assembly at a relatively low temperature; (d) controlling an atmosphere so as to nonoxidize the chamber; (e) thermally softening the glass block by raising the temperature of the die; (f) applying a pressure to the die; (g) setting the temperature of the glass to a temperature which is equal to or lower than a glass transformation point by cooling the die; (h) eliminating the pressure from the die; (i) further reducing the temperature of the die; and (j) taking out the finished lens.
According to the glass press-molding method, a glass preform which was heated at a high temperature is inserted into the die which was heated at a high temperature and is press-molded and, thereafter, it is cooled and an optical device is manufactured. Therefore, a thermal fatigue or the oxidation of the die occurs due to a thermal shock thereof, which makes the life of the die short. The molding surface of the die must be cleaned. Since, a mist of the glass surface occurs due to the reaction of the glass and the die material.
As a countermeasure for the above problems, there has been proposed JP-A-61-31321 in which the molding surface of the press-molding die is formed with ceramics or JP-B-61-32263 (U.S. Pat. No. 4481023) in which the surface of the die is formed with tungsten carbide, an alloy of noble metal, or the like.
On the other hand, as a countermeasure to improve the separating efficiency between the glass and the die which were heated at high temperatures, there has been proposed JP-A-61-197430, which discloses:
a die for molding an optical glass device in which a molding surface of a predetermined shape is formed on the die to press-mold a desired molding object, characterized in that a coating film consisting of AlN is formed on the molding surface; PA1 an optical glass device molding die which is formed by a metal material of either one of an Ni base alloy, an Fe base heat resistance alloy, and a WC--Co system alloy; and PA1 an optical glass device molding die which is formed by a ceramic material containing either AlN or Si.sub.3 N.sub.4 as a main component.
However, 13 Cr martensite steel has drawbacks such that it is easily oxidized and Fe is diffused into the glass at a high temperature and the glass is colored. Although SiC and Si.sub.3 N.sub.4 are generally considered as being hardly oxidized, they are also oxidized at a high temperature and an SiO.sub.2 film is formed on the surface, so that adhesion with the glass occurs. Further, since their hardnesses are high, the working efficiency of the die itself is bad. When tungsten carbide is used as a material of the die and the glass preform is press-molded, a reaction occurs between the glass surface and the surface of the die due to the press-molding by the heating at a high temperature and a phenomenon such that the glass reactant is deposited onto the surface of the die when the molded glass is separated from the die occurs. The inventors of the present invention have found out that such a reaction phenomenon occurs in the following manner. The sintered hard alloy is formed by coupling tungsten carbide by using cobalt Co as a binder. When tungsten carbide and glass are heated at a high temperature and are press-molded, cobalt on the surface of the tungsten carbide and lead contained in the glass react as follows. EQU Co+PbO.fwdarw.CoO.sub.x +Pb
On the other hand, tungsten carbide WC and lead contained in the glass react as follows. EQU WC+PbO.fwdarw.WO.sub.x +Pb+CO+CO.sub.2
Thus, lead in the glass is precipitated and causes a product between the die and the glass, so that a phenomenon such as a mist as mentioned above occurs when the glass is separated from the die. If the surface of the die is made of a ceramics material, a reaction between the ceramics and lead oxide in the glass hardly occurs. Therefore, a product due to the reaction as in the case of using tungsten carbide mentioned above is hardly produced. However, in place of it, the contact force between the surface of the die and the surface of the glass molded article increases. Since such a contact force is larger than the strength of the glass when it is separated from the die, the surface of the glass molded article is destroyed and a cracking phenomenon of the glass molded article is caused.
It is the first object of the invention to propose a manufacturing method in which even a high endurance material having a high adhesion tendency can be used as a material of the die.
Practically speaking, it is an object of the invention to propose an optical device manufacturing method in which a material which can reduce a reaction such that cobalt and tungsten carbide in a sintered hard alloy directly reduce lead oxide in the glass and which has a separation effect is interposed between the die surface and the glass surface, so that no adhesion occurs and the die surface hardly deteriorates.
Still another object of the invention is to propose a manufacturing method in which a very small amount of material which actively reacts to lead in a glass is interposed between the die surface and the glass preform, thereby blocking a direct reaction between the die and the glass preform.
In the above invention, the inventors of the invention propose a manufacturing method in which when a die and a glass preform are heated at a high temperature and are press-molded, an atmosphere in which the press-molding is executed is filled with a gas containing hydrocarbon (practically speaking, a mixture gas of a nonoxidizer gas and a hydrocarbon gas) and a reaction such as EQU C+PbO.fwdarw.Pb+CO+CO.sub.2
is caused between carbon in the gas and lead in the glass, thereby blocking a reaction between the die and the glass.
The inventors of the invention also propose an application example in the case of using a preferable atmosphere. For instance, in the case of using a process to fill the whole molding steps by an inert gas such as a nitrogen gas or the like which is suitable for solving various problems such as thermal stress due to high temperature and high heat of a mechanism which is necessary for the operations in the steps which are needed for a molding apparatus, particularly, for a glass press-molding included in the molding apparatus. For instance, the steps such as conveyance, insertion, heating, and cooling of a die and a glass preform, fetching of a molded article, and the like, fatigue due to the repetition of the heating and cooling cycles, and the like.
Further, the inventors of the invention propose a manufacturing method in which a ceramics sintered material as a die material which does not cause any reactant with lead in a glass is used and a gas containing carbon is interposed between the die made of the ceramics sintered material and a glass preform, thereby causing a reaction between carbon in the gas and lead in the glass, so that it is possible to avoid the cracking phenomenon of the glass molded article which is caused by an increase in contact force due to the ceramics die.